1. Field of the Invention
This invention relates to a method for producing a steel tube, in which a metal sheet or coil is formed in a bending process to a tubular body having a round cross section, is welded in an ensuing welding process along the longitudinal edges facing one another to produce one continuous seam, and is then subjected to a stress-relieving treatment.
2. Discussion of Related Art
One method of this type is described in German Patent Disclosure DE 10 2006 010 040 B3. In this known method, the tube is compressed by a straightening machine from the outer circumference, by a plurality of welding devices, offset in a circumferential direction and located at an identical location in the axial direction, for concentric straightening. The welding devices have straightening shells adapted to the shape of the outer cross section of the tube. The straightening shells can be driven, for instance hydraulically, individually or in dependence on one another, and the actuation can be done by open-loop or closed-loop control. Via the closed-loop control axes, the straightening cylinders, with the straightening shells, can straighten the tube until its contour is circular, and the calibration is done with respect to the diameter and/or the ovality. Upsetting of the material past the elongation limit is also possible with what is called here for the first time impansion.
European Patent Disclosure EP 0 438 205 A2 shows a method and an apparatus for straightening the ends of elongated workpieces. With the workpiece at a standstill, at least one cross section, sought in the end region, is subjected to an alternating increasing and decreasing bending stress, and a predetermined maximum sag extends around the workpiece axis once or multiple times. An alternating increasing and decreasing bending stress is selected so that the cross section sought is deformed into the plastic range. To generate a deflection of the workpiece axis into an orbit past or beyond the limit of elasticity of the workpiece there are at least three tappets, movable in the radial direction and disposed symmetrically about a common axis, which are each connected to a travel- and time-dependently controllable piston-cylinder unit. The tappets, as a result of a controlled linkage of the piston-cylinder units to one another, execute a sinusoidal reciprocating motion in phase-offset fashion during the straightening process. In this case, straightening is not effected with regard to roundness or ovality but rather a correction is made of deviations in rectilinearity of the crooked ends, such as longitudinal straightening.
With a straightening machine for tubes shown in French Patent Disclosure FR 737 123 A, these tubes are also straightened in their longitudinal direction, specifically in the warm state. Here, two opposed straightening elements, which between them receive the tube and can be pressed against one another by a lever mechanism with a drive, extend over an entire length of the tube. The straightening elements are for example rounded in accordance with the diameter of the tube, and the inner part of the straightening elements can be replaceable. Before the straightening process, the tubes are heated to the red-hot state and are evacuated. After the performed longitudinal straightening, the tubes are delivered by an ejector to a cooling device. Straightening large steel tubes, in particular, by such methods or provisions is complicated, and problems and solutions for concentric straightening are not found in this reference.
In German Patent Disclosure DE 196 02 920 A1, a method for producing tubes, in particular large tubes, is disclosed in which the tubes are calibrated and straightened by cold widening or expansion after the seam welding on the inside and the outside.
German Patent Disclosure DE 41 24 689 A1 shows a method and an apparatus for eliminating shape errors and diminishing harmful intrinsic stresses in the longitudinal seam of welded extruded tubes, also by widening the tube, for which purpose a widening mandrel located on the inside is employed. The widening of the extruded tube is done to such an extent that intrinsic stresses present in the circumferential direction are intended to be diminished as much as possible.
In straightening tubes, nonuniformities in the tube shape, such as local ovality on the tubular body, are corrected by local shaping of material. Stress is not diminished uniformly by way of the tube jacket, in particular the tube circumference. Instead, additional undefined stresses are generated in the material by the known local ovality corrections. Although a target diameter can be established through a relatively great effort in this way, with the straightening, a uniform upsetting strength of the material over the circumference of the tube, in particular, fails to be achieved.
In the expansion method, the tools generate a uniform force on the inside of the tube, and in the concentric straightening, this puts the material uniformly into a circular shape. In this operation, however, unfavorable stress states can be created in the tubular body, and as a result the upsetting strength and hence the resistance to collapsing of the pipeline may lessen. In coated tubes, so-called clad tubes, damage to the material can also occur, so that such tubes can often not be calibrated by this method. Such adverse effects can be further amplified with an increasing degree of expansion.